The proposed research will test the hypothesis that cyanide is an important physiological regulator of nitrate assimilation. Further objectives will be: to determine, by the method of EPR, the mechanism by which cyanide inactivates the enzyme, nitrate reductase, to determine the in vivo mechanism of activation of nitrate reductase and to obtain detailed information on the quaternary structure and prosthetic groups of assimilatory nitrate reductases from plant, algal, fungal and bacterial sources.